Observing the milky way, part I: Sagittarius & Scorpius.
The largest single object in the sky is the Milky Way, which makes a full 360[degrees] circuit around the heavens. But it isn't a featureless, hazy band; it has bays, rifts, and star clouds that can be seen easily by the unaided eye and are often quite spectacular in binoculars -- as long as you view them from a dark location on a clear, moonless night. These features, as well as the distribution of the brightest open clusters and nebulae along the Milky Way, reflect our galaxy's spiral structure in the neighborhood of the Sun.
We see the Milky Way in two dimensions, as though it were painted on the celestial sphere. But this view is misleading; objects at very different distances often appear side by side or even superposed. That's especially true in the constellations Sagittarius, Scorpius, and Ophiuchus, where several spiral arms lie between us and the center of the galaxy. This article is meant to give you a sense of depth perspective when you view this area with binoculars or your unaided eyes. It will be followed by two more articles describing the remainder of the Milky Way band and how its appearance reflects our galaxy's structure.
Professional astronomers have nearly as much trouble as amateurs in discerning our galaxy's structure; it's difficult to understand a forest when you're surrounded by its trees. The broad outlines have been known or suspected for many decades, but the details are still controversial.
The Milky Way Galaxy seems to be an SBc spiral with fairly loosely wound, luminous spiral arms and a relatively small central bulge. The galaxy's disk is probably not quite 100,000 light-years in diameter, and the Sun is between 25,000 and 29,000 light-years from the center. The spiral arms are less than 1,500 light-years thick when viewed edge on, but they're embedded in a thick disk of more smoothly distributed stars some 3,000 to 4,000 light-years thick. The galaxy's central bulge is something of a flattened spheroid, with a polar diameter of roughly 8,000 light-years and an equatorial diameter around 10,000 light-years. In both the central bulge and the disk, star densities taper off gradually, so these dimensions cannot be specified with precision.
The bulge is embedded in a weak central bar whose long axis is oriented between 10[degrees] and 40[degrees] from our line of sight -- making it difficult to detect. Some researchers think there are two superposed bars.
Our Milky Way Galaxy is well above average in terms of size, luminosity, and mass. For instance, our Local Galaxy Group contains only one other galaxy of similar size (Messier 31), one that's somewhat smaller (Messier 33), and several dozen that are much smaller.
The Milky Way contains well over 100 billion stars and probably well under a trillion -- most of them considerably less massive and much less luminous than our Sun. Its total luminosity is at least 15 billion Suns, corresponding to an integrated absolute magnitude of -20.5.
Although stars account for almost all of our galaxy's light, they make up only a fraction of its mass. A roughly comparable mass exists in the form of interstellar gas (mostly hydrogen and helium) and "dust" (microscopic particles). Dust accounts for less than 1% of the mass, but it has a disproportionate effect on our galaxy's appearance and evolution because it blocks visible light, whereas hydrogen and helium are transparent at most wavelengths.
The Galactic Center
Astronomers have long suspected that the center of our galaxy lies near the junction of the constellations Sagittarius, Scorpius, and Ophiuchus. That's both because this is the brightest and broadest part of the Milky Way band and because of the distribution of globular clusters. Of the 29 Messier globulars, seven are in Sagittarius, three in Scorpius, and six in Ophiuchus, compared to just one (M79 in Lepus) in or near the winter Milky Way.
The suspicion was confirmed when radio observations detected a strong source at right ascension 17h 45.7m, declination -29[degrees] 00', a visually unremarkable spot about 5[degrees] west-northwest of Gamma ([gamma]) Sagittarii, the
Western-most star in the Teapot asterism. Several lines of evidence indicate that these radio emissions come from an accretion disk spiraling around a supermassive black hole at the galaxy's center.
The galactic nucleus -- the area around the center -- can be studied only at radio, microwave, and infrared wavelengths because the dust of the galaxy's interior spiral arms, and the dense dust around the nucleus itself, blocks virtually all radiation at shorter wavelengths. Visible light from the nucleus is obscured roughly 30 magnitudes--a factor of one trillion-- by intervening matter.
The nucleus is something like an extreme globular cluster, crowding more than 100 million stars into a sphere some 150 light-years in radius. (By contrast, there are only about 50,000 stars within 150 light-years of the Sun.) Deep inside the nucleus are three compact open clusters of rapidly evolving giant and supergiant stars mingled with dense gas and dust.
The Central Bulge
Despite the heavy dust clouds in the central regions of our galaxy, we can see four layers of galactic structure when we look toward Sagittarius and Scorpius.
The Great Sagittarius Star Cloud is the innermost galactic structure that can be observed in visible wave-lengths and the most distant Milky Way structure that can be seen with the unaided eye. It stretches several degrees north from Gamma and Delta ([sigma]) Sagittarii and is a splendid sight in small binoculars -- a bright glow with multitudes of momentarily resolved star-sparks. It is in fact a section of the Milky Way's central bulge.
The central bulge is depleted of the gas and dust from which new stars form, so unlike the spiral arms it contains no bright, young, blue stars. Instead, its bright-est stars are K-type orange giants. So on color photos the Great Sagittarius Star Cloud has a yellowish tint.
Most of our galaxy's bulge is hidden from our view by the dust of the inner spiral arms. If we could see the whole bulge, it would stretch from the Stinger of Scorpius to the Small Sagittarius Star Cloud (Messier 24) and reach more than halfway to Antares, as shown on the preceding page. The Great Sagittarius Star Cloud is visible only because of a rather large window through the interstellar dust of the galactic interior.
Several other similar windows allow us to see relatively long distances across our galaxy's spiral disk. Another window through which we glimpse a segment of the galaxy's central bulge is in the direction of the foreground open cluster Messier 7 in the Tail of Scorpius. On color photos the little star cloud around M7 has the same yellowish tone as the Great Sagittarius Star Cloud to its north. Thus we see M7, roughly 1,000 light-years away, superposed on the much more distant galactic bulge.
The Norma Arm
Another window through the interstellar dust of the galactic interior gives us a view of the second most distant galactic structure visible in Sagittarius: the Small Sagittarius Star Cloud, also known as Messier 24. It's a roughly rectangular glow stretching northeast to south-west, measuring about 2[omicron] x 3/4[omicron], and located 2[omicron] north-northeast of Mu ([micro]) Sagittarii. In binoculars it's not as bright as the Great Sagittarius Star Cloud, but it's more richly sprinkled with 7th- to 10th-magnitude stars. M24's estimated distance of 10,000 to 16,000 light-years implies that it's a stretch of one of the deep interior spiral features of our galaxy. Its binocular appearance confirms this, because spiral arms have many highly luminous super-giants that should be easily resolvable in small instruments even at M24's distance.
M24 is probably part of the Norma Arm, the second spiral arm in from our own Orion-Cygnus Arm. Some researchers call this the Scutum-Centaurus Arm and use the term Norma Arm for a smaller arm that lies closer to the galactic center. This is the terminology adopted in the galaxy diagram on the foldout Milky Way map in the center of this magazine.
The bright Norma Star Cloud in the far southern Milky Way and the Scutum Star Cloud north of Sagittarius probably also lie in the Norma Arm. We will discuss them in future articles.
The Sagittarius-Carina Arm
Moving outward from the Norma Arm toward the Sun, the next spiral feature is the Sagittarius-Carina Arm -- so named because many major bright emission nebulae and open clusters are distributed along it from Sagittarius to Carina. From northeast to southwest these include M16 (the Star Queen or Eagle Nebula) in Serpens; the emission nebulae M17 (Swan), M20 (Trifid), and M8 (Lagoon) in Sagittarius; the open clusters M21 in Sagittarius and NGC 6231 in Scorpius; the open cluster and emission nebula NGC 6193 and 6188 in Ara; the open cluster NGC 4755 (Jewel Box) in Crux; and the giant emission nebula NGC 3372, also known as the Eta Carinae Nebula. All of the emission nebulae contain embedded clusters and/or associations of young stars, though in some cases these stars are heavily obscured by dust.
Although it's rather far south for observers in the northern United States and Europe, NGC 6231 in the Tail of Scorpius, a major tracer of the Sagittarius-Carina Arm, is well worth viewing in binoculars. It's a special cluster, one of the richest concentrations of extremely hot and luminous O-type giants and supergiants known in our galaxy. An outlying member of the cluster is the 4.8-mag-nitude star Zeta (1) ([xi]1) Scorpii, 1/2[omicron] to its south. Zeta (1) is a B1.5 Ia+ extreme supergiant with an absolute magnitude around -8.8 (a luminosity of almost 300,000 Suns).
Centered about 1[omicron] north-northeast of NGC 6231 is a rich binocular field of 6th- to 9th-magnitude stars that is catalogued as the open cluster Collinder 316 or Trumpler 24: it's in fact the richest outlying part of Scorpius OB1, the vast stellar association of which NGC 6231 is the core. This area is also discussed in Binocular Highlights on page 40.
The Sagittarius-Carina Arm clusters and nebulae in Sagittarius and Scorpius are between 4,500 and 7,000 light-years distant, suggesting that in this direction the arm is centered about 5,500 light-years from us. Further northeast along the arm, M16 in Serpens is somewhat more distant, about 6,500 light-years. Here the Sagittarius-Carina Arm begins its arc in toward the galactic interior. The arm's incurving edge is at the Scutum Star Cloud, where the Sagittarius-Carina and Norma arms may intersect. On the opposite, southwestern end of the Sagittarius-Carina Arm, the Eta Carinae complex is about 8,000 light-years away. But other Sagittarius-Carina Arm associations have been identified beyond it, because here the arm is beginning to curve out to the galactic exterior and we have a long view down its outcurving length.
The Orion-Cygnus Arm
The fourth feature of galactic structure visible toward Sagittarius is of course our own Orion-Cygnus Arm. Its inner edge is marked by the Great Rift chain of dust clouds from Deneb on the northeast to Alpha Centauri on the southwest. The Great Rift will be described in more detail in the next installment of this article.
The inner edge of the Orion-Cygnus Arm is also traced by the Scorpius-Centaurus Association, which includes the majority of the bright stars from Scorpius on the north-east, through Lupus and Centaurus, to Crux on the south-west. This association is centered about 550 light-years away in a direction between Alpha ([alpha]) Lupi and Zeta Centauri, and is highly elongated, being 700 light-years long, 250 light-years "high" (perpendicular to the galactic plane), and 400 light-years deep along our line of sight. It's slightly nearer than the Great Rift chain of dust clouds, though Antares and a couple of other stars in the Head and Heart of Scorpius are just within the nearest fringes of Great Rift dust. Except for the M-type red supergiant Antares, all the bright stars of the Scorpius-Centaurus Association are blue B0, B1, and B2 main-sequence and giant stars. In binoculars the color contrast between ruddy-orange Antares and the silver-blue Sigma ([sigma]) and Tau ([tau]) Scorpii to its west-northwest and southeast is absolutely stunning. All three stars fit in the same binocular field of view.
Binoculars also show the bright (5.7-magnitude) globular cluster M4 just 1.3[degrees] west of Antares. In 10x50 glasses, M4 appears as a large hazy patch (June issue, page 45). It's one of the two or three nearest globulars to us, roughly 7,200 light-years away. This is about the same distance as the Sagittarius-Carina Arm, but globular clusters are far too old to be true spiral arm tracers. Spiral arms are thought to change fairly rapidly, perhaps even disappearing and reforming over billion-year time frames. Most of the Milky Way's globular clusters, by contrast, seem to be nearly as old as the galaxy itself.
Moreover, M4 is a good way off the Sagittarius-Carina Arm, about 16[degrees] northwest of its core. So observers in M4 would have an excellent view "down" into the Sagittarius-Carina Arm. And because M4 is well outside the dust clouds that lie along the center of the Milky Way, any observers in the cluster would have a far better view than we do of the galaxy's central bulge.
In the next two articles of this series we will visit our galaxy's cardinal points to further our sense of depth perspective on our place in the Milky Way Galaxy.
Photo by Babak Tafreshi
Craig Crossen is a professional editor living in Vienna, Austria. He is coauthor with Gerald Rhemann of the book Sky Vistas: Astronomy for Binoculars and Richest-Field Telescopes.
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|Title Annotation:||Galactic Depth Perspective|
|Publication:||Sky & Telescope|
|Date:||Jul 1, 2013|
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